1. Field of the Invention
The present invention relates to an inner tube for CVD apparatus used for the production of semiconductors. More particularly, the present invention relates to an inner tube for CVD apparatus which effectively prevents the occurrence of particles which causes contamination to silicon wafers.
2. Description of the Related Art
The production of semiconductor devices conventionally relies on the so-called chemical vapor deposition method (CVD method for short hereinafter) which brings about gas-phase chemical reactions of more than one kind of compound gas to deposit silicon, silicon nitride, etc. in thin film on wafers. The CVD method employs a fixture called an inner tube which is so arranged as to surround silicon wafers as shown in FIG. 1. It regulates the flow of reactant gases and keeps the wafer temperature uniform.
Since the CVD method for the production of silicon semiconductor devices handles corrosive reactant gases (such as SiH4) at high temperatures, the above-mentioned inner tube needs good heat resistance, corrosion resistance, and mechanical strength. The inner tube meeting these requirements has conventionally been made of quartz.
The production of semiconductor devices needs a clean environment to avoid contamination with impurities as far as possible. However, this requirement is not fully met by the conventional CVD method mentioned above, which consists of feeding a reactant gas from a gas inlet nozzle 5 into a sealed container in which are placed wafers 3 as shown in FIG. 1. In the sealed container, CVD takes place not only on wafers but also on the inside surface of the inner tube 1. (Those materials constituting the deposited film on the inner tube are referred to as CVD film constituents hereinafter.) For efficient production, the inner tube carrying CVD film constituents is used repeatedly without being renewed. After repeated use, CVD film constituents gradually accumulate to such an extent that they peel off from the inner tube and stick (in the form of fine particulate impurities) to wafers, thereby decreasing yields.
The conventional way to prevent the occurrence of such particles is to remove deposits by periodical cleaning of the inner tube with a chemical solution such as hydrofluoric acid and nitric acid. Frequent cleaning operations lower productivity and increase production cost. Moreover, cleaning with hydrofluoric acid reduces the life of quartz inner tubes because of its vigorous corrosive action. This necessitates frequent replacement and hence increases production cost.
New inner tubes are made of silicon carbide so that they resist not only corrosive gases such as SiH4 used for the production of semiconductor devices but also cleaning chemical solutions. However, there still remains the unsolved problem with particles arising from CVD film constituents.
The inner tube used to produce semiconductors by the CVD method needs not only good heat resistance and corrosion resistance but also a specific inside surface which prevents CVD film constituents from peeling off, thereby reducing the frequency of maintenance work to remove deposited film.
With the foregoing in mind, the present inventors carried out researches on the production of inner tubes from glass-like carbon having good performance. As the result, it was found that a tube of glass-like carbon having good heat resistance and corrosion resistance can be obtained by the following process. First, a thermosetting resin is prepared as a precursor of the glass-like carbon. This thermosetting resin has such curing characteristics that it reaches a degree of cure of 10% at 115xc2x0 C. within 5-60 minutes (T10). It also exhibits such fluidity that it flows more than 60 mm at 100xc2x0 C. in the disk flow test according to JIS K6911. Then, this thermosetting resin is made into a tube by centrifugal molding, and the resulting tube is carbonized. A patent application for this invention has been applied.
In their continued research, the present inventors found that the inner tube of glass-like carbon used in the CVD apparatus for semiconductor production should have specific physical properties so that it will not contaminate silicon wafers with microparticles peeling off from CVD film constituents which have accumulated on the inside surface of the inner tube during the CVD process.
The present invention was completed to tackle this problem. It is an object of the present invention to provide an inner tube for CVD apparatus which possesses not only good heat resistance and corrosion resistance but also specific physical properties contributing to good adhesion between its inside surface and CVD film constituents, thereby suppressing the occurrence of particles and the contamination of silicon wafers with particles.
The present invention is directed to an inner tube for CVD apparatus which is made of glass-like carbon. The inner tube possesses at least one of the following four characteristic properties. The present invention is directed also to a carbon-based inner tube for CVD apparatus which is coated with a film of glass-like carbon. The inner tube also possesses at least one of the following four characteristic properties.
(1) The inner tube has a coefficient of thermal expansion ranging from 2xc3x9710xe2x88x926 to 3.5xc3x9710xe2x88x926.
(2) The inner tube has an inside whose surface roughness is 5-100 nm (measured according to JIS B0651 and JIS B0601).
(3) The inner tube has an inside surface on which the oxygen/carbon atomic ratio (O/C) is 0.04-0.4 (measured by X-ray photoelectron spectroscopy).
(4) The inner tube has an inside surface which gives an I(D)/I(G) ratio ranging from 0.8 to 1.4 in Raman spectroscopy. The I(D)/I(G) ratio is an index showing the chemical structure of the inside surface, with I(D) being a peak intensity representing the Cxe2x80x94C bond of diamond-like structure and I(G) being a peak intensity representing the Cxe2x80x94C bond of graphite-like structure.